Sulfonic acid ring-substituted polyaniline is a "self-doped" conducting polymer, reported by Yue, Epstein and MacDiarmid in Proc. Symposium on Electroresponsive Molecular and Polymeric Systems, Brookhaven National Laboratory, Oct. 1989, to have a conductivity of .about.0.03 S/cm. without external doping. Synthesis of the material is also described in J.A.C.S. 1991, V.113, N.7 pp. 2665-2671 which shows a conductivity of .about.0.1 S/cm measured on pressed pellets.
Fibers of a blend of polyaniline and poly(p-phenylene terephthalamide) prepared from homogeneous solutions in 98% by weight sulfuric acid are described in Polymer Commun. 31,275 (1990). The externally doped fibers are said to have improved mechanical properties while retaining the conductivity of pure polyaniline. The concentration of poly(p-phenylene terephthalamide) in the spinning solution employed by the experimenters was below the onset of formation of lyotropic phase, thus, the fibers were spun from isotropic solutions.
MacDiarmid et al., U.S. Pat. No. 5,177,187 taught a process for preparation and spinning of amorphous, 100% by weight polyaniline fibers from solutions of the polymer in concentrated sulfuric acid or N-methyl pyrrolidinone, but the conductivity of the fibers taught by MacDiarmid was achieved by coagulating the fiber in solutions containing hydrochloric acid which served as the external dopant.
Blends of polyaniline and poly(p-phenylene terephthalamide) have been taught by Jen et al. in U.S. Pat. No. 5,069,820 (melt spinning), Smith et al. in U.S. Pat. No. 5,196,144 (solution spinning from 96% sulfuric acid solutions) and Elsenbaumer in U.S. Pat. No. 5,160,457 (spinning from solutions and doping).
The inventors of the present invention wanted a method to prepare high strength, high modulus conductive fibers of consistent and permanent conductivity. In order to achieve such a fiber product they found that only certain sequences of spin dope preparation steps and the use of specific concentrations of sulfuric acid, that is those in excess of 98% by weight, in preparing the spinning solutions resulted in the insitu ring-sulfonation of the polyaniline and the desired mechanical and electrical properties of the fibers.